Two major trends in data processing machines have been toward greater computational power and smaller physical size. The first trend is driven by the increasing amount and types of data which are handled by computers and computer networks. Faster computers are continually sought in order to solve complex numerical problems and to process an ever increasing data load. The second trend is driven in part by the desire to have portable devices--laptop, notebook, and even handheld computers.
Some of the limitations facing computer designers as they address these two goals are related to circuit power dissipation. High speed, high integration density chips dissipate considerable power, more than can be removed through conventional packaging technology. In addition, a typical office computer is becoming limited in its computational capability by the total power available from the standard office outlet. The operating time of portable machines is limited by the amount of energy which may be stored in the batteries. These batteries are often the heaviest component of the device, yet can only provide several hours of active use.
Solutions to the above problems have been to either improve the heat removal system for the integrated circuits or to reduce the dissipation. In order to keep high performance integrated circuits operating at a reasonable temperature, workers have developed advanced packaging concepts with active or passive cooling mechanisms. Active cooling can be attractive for mainframe and supercomputer applications where cost and size are secondary considerations. Cost is more of a concern in midrange office computers however, as are fan noise and the load on air conditioning system. There is a trend to lower the power supply voltage as the line widths in integrated circuits get smaller, and this will help to reduce the overall power dissipation. In addition, portable computers have recently incorporated innovative designs which turn off inactive portions of the machine, such as disk drives, in order to save energy.
Contemporary logic circuits can be divided into two classes, those which dissipate power continuously and those which only dissipate during logic state transitions. Emitter Coupled Logic is an example of the first class; each logic stage draws a DC current that is always flowing through one or another switching transistor. Another logic style which dissipates static power is ratioed nMOS. Current flows continuously through the output transistor and load when a gate is in the false state. Complementary MOS (CMOS) and precharged MOS logic styles are examples of the second class of logic circuits. They do not require power input between changes of state; energy is only dissipated when changing a logical cell's output voltage.